The inventive subject matter relates to HD FM apparatus and methods of operating the same and, more particularly, to diversity delay error compensation of and for FM HD audio processors.
HD Radio™ is an in-band on-channel digital radio technology in which a broadcaster transmits audio and data using a digital signal transmitted in the same spectrum as the broadcaster's standard analog FM signal. Stations typically simulcast digital and analog audio signals, which are received by receivers that can “blend” audio in the received signals to produce an audio output.
FIG. 1 illustrates a conventional HD radio transmission system. Studio or other input audio is provided to an audio processor 10, which typically provides processed analog FM audio and HD FM audio for transmission by a transmitter 30 via an IP network 20. The audio processor 10 may be designed to limit overmodulation, compensate for non-linearities in the transmitter 30 and adjust overall loudness to a desirable level.
There are significant and variable latencies in this arrangement, including latencies in processing by the audio processor 10. There may also be significant latencies in conveying the HD audio content to the transmitter 30, which may be remote with respect to the audio processor 10 and subject to congestion, retransmission and other effects arising from the use of the IP network 20 to convey the audio content. These latencies often result in the transmitted HD FM signal lagging the transmitted analog FM signal by a significant amount of time, e.g., 8-10 seconds. Accordingly, FM stations typically delay their analog FM signals so that FM/HD receivers can nearly seamlessly switch between the two signals under low signal strength and/or high interference conditions. In a typical FM/HD station's signal chain, transmission of the FM analog signal through the audio processor is usually delayed by several seconds, with the specific amount being dependent upon the station's chosen FM/HD hardware and the studio-to-transmitter audio program transport mechanism(s) which may be in use.
In the early days of HD technology, when stations typically installed their FM/HD on-air audio processors at the transmitter site, typically the only signal transmitted between the studio and transmitter was a single stereo program channel. In this arrangement, the separately processed FM and HD program audio came directly from the on-air processor's outputs, was fed directly into the FM/HD transmission equipment, and once diversity delay was set to the required amount, it typically needed little or no subsequent readjustment. Years later, the introduction of lower cost, high bandwidth, bidirectional IP-based microwave systems allowed stations to relocate the on-air audio processor to the studio and send the separate analog FM and HD FM audio signals as separate data packet streams over an IP based link to the transmitter site miles away.
The problem with IP links, however, is that the timing relationship between the analog FM and HD FM audio packets is generally indeterminate. By relocating the audio processing to the studio, the diversity delay can vary widely and can drift in and out of tolerance due to the IP-based link.
A few techniques have been developed to address this latency problem. Several years ago, manufacturers of FM modulation monitors began to incorporate the ability to perform diversity delay error measurements in their products. Station engineers typically manually adjusted diversity delay to reduce timing errors, but such modified modulation monitors do provide a more reliable scheme for measuring timing errors than the old way of “tuning it by ear.” These modified FM modulation monitors soon included the capability to communicate with audio processors, such that the modulation monitor could control the audio processor's diversity delay in closed-loop fashion, such as over an IP network. FIG. 2 illustrates such an arrangement wherein an FM/HD modulation monitor 40 provides a diversity delay correction command to an audio processor 10, which responsively adjusts a delay the audio processor 10 applies to an analog FM audio signal transmitted to the transmitter 30. Examples of such modulation monitors include the FMHD-1 FM HD Stereo Monitor/Analyzer produced by Belar Electronics Laboratory, Inc., and the Series 2 M4 TimeLock™ Broadcast Receiver produced by DaySequerra Corporation.
Another technique for compensation for dealing with diversity delay errors is illustrated in FIG. 3. In this arrangement, a delay processor 50 is connected at the output of the audio processor 10. The delay processor 50 measures a time discrepancy between analog FM and HD FM signals transmitted by the transmitter 30, and delays the analog FM output of the audio processor 30 accordingly. An example of such a delay processor is the Justin 808 HD Radio™ Delay produced by Innovonics, Inc.
Potential disadvantages of both of the aforementioned arrangements include the additional cost of the FM/HD modulation monitor or delay processor (typically several thousands of dollars), and an additional potential point of failure in the station's audio chain. In addition, a delay processor such as that shown in FIG. 3 may be limited to correcting only a limited amount of diversity delay error (e.g., one second). Accordingly, there is an ongoing need for improved techniques for diversity delay error compensation.